The present application relates generally to signal processing, and more specifically to systems and methods of increasing the accuracy of time delay estimates of signals propagating through an environment.
Various industrial and scientific techniques require accurate estimations of time delays of signals propagating through an environment such as an underwater environment, a soil environment, or an environment comprising living tissue. For example, in an underwater environment, sonar systems may be employed to estimate time delays of sonar pulses reflected from an object or target to estimate a distance to the target (also known as estimating the range of the target). Conventional systems for performing sonar range estimation typically transmit one or more sonar pulses (“pings”) comprising sonic or supersonic pressure waves toward a selected target, and receive one or more sonar pulses reflected from the target. Such reflected sonar pulses (“echoes” or “returns”) may include a significant amount of background noise and/or other interfering signals in addition to the reflected sonar signals of interest. For example, a conventional sonar system may comprise a coherent receiver including a cross correlator configured to receive the echo and a representation of the transmitted sonar pulse or ping, which are cross-correlated within the coherent receiver to generate a peak cross correlation value. The conventional sonar system typically compares the peak cross correlation value to a predetermined threshold value. If the cross correlation value is greater than the predetermined threshold value, then the reflected sonar signal of interest has been successfully detected. The conventional sonar system may then utilize the cross correlation peak to obtain a measure of the range of the target.
One drawback of the above-described conventional sonar system is that the level of background noise and/or other interfering signals contained within the echo or return may be sufficient to cause the reflected sonar signal to go undetected or to be falsely detected, thereby causing the cross correlator to produce inaccurate range measurements. Such inaccurate range measurements are likely to occur in low signal-to-noise ratio (SNR) sonar environments, in which the noise power within the echo may be comparable to or greater than the reflected signal power. This can be problematic in sonar ranging systems because a reduction in the measurement accuracy of the cross correlator typically leads to a concomitant reduction in sonar ranging accuracy.
Prior attempts to increase the accuracy of sonar ranging have included filtering out at least some of the background noise before providing the echoes to the cross correlator. However, such attempts have generally not worked well enough to allow successful detection of reflected sonar signals and accurate estimation of range in low SNR sonar environments. This is due, at least in part, to the fact that sonar systems typically receive echoes that include various types of noise from a variety of different noise sources. For example, a sonar system may transmit pings through a medium such as water from a ship or submarine that produces noise across a wide frequency range. Further, other ships, submarines, or structures producing noise across wide frequency ranges may be within the vicinity of the sonar system. Moreover, the natural interaction of the water and objects within the water including the selected target may produce a substantial amount of ambient noise.
In addition, sonar ranging systems may receive echoes from multiple selected (and unselected) targets, each target having its own associated noise level, and it may be desirable to determine the noise level and range of each target separately. Such noise associated with multiple targets may be stationary or non-stationary, linear or nonlinear, or additive or non-additive. Further, at least some of the background noise may result from reverberations and/or random signal distortions of the ping and/or echo, and therefore the noise level and its structure may be significantly affected by the transmitted sonar signal. However, conventional sonar systems are generally incapable of accurately estimating noise levels and target ranges in the presence of non-stationary, nonlinear, non-additive, and/or signal-dependent noise.
Moreover, the density and temperature of the transmission medium (e.g., water) and the frequency of the transmitted/reflected sonar signals may affect the decay rate of the sonar pulse propagating through the medium. In addition, the absorption of certain frequencies of the ping by the target may affect the strength of the resulting echo. However, conventional sonar systems are generally incapable of fully compensating for such factors when called upon to generate accurate noise and range estimates.
It would therefore be desirable to have a system and method of increasing the accuracy of time delay estimates of signals propagating through an environment that avoids the drawbacks of the above-described systems and methods. Such a system would have increased resilience to noise, thereby allowing an increase in the operating range of the system and/or a decrease in the power level of signals employed by the system.